It is well known that currently, the automobile braking system usually adopts a vacuum booster to achieve the purpose of a boosting in braking. The main principle is to realize the boosting function using the pressure difference between the vacuum and atmosphere. FIGS. 1 and 2 illustrate a structure diagram of an existing vacuum booster, and an enlarged structure diagram of Part A in FIG. 1, respectively. The existing vacuum booster 100 mainly includes front and rear housing assemblies, a control valve body 101, an air valve 102, a rubber reaction plate 103, a reaction rod 104, etc. In which, the rubber reaction plate 103 is tightly clamped between the control valve body 101 and the reaction rod 104, a base plate 105 of the reaction rod 104 is closely engaged with one side of the rubber reaction plate 103, an end face of the control valve body 101 is closely engaged with the other side of the rubber reaction plate 103, so as to wholly clamp the rubber reaction plate 103. During the usage, by means of a thrust generated by different pressures in two working chambers of the vacuum booster, the braking force is amplified in proportion to achieve the braking effect. However, the above existing vacuum booster is a single-rate brake vacuum booster, and the rate is constant in the braking process. As a result, when a high deceleration rate is required (e.g., emergency braking), a large braking force shall be provided, thus the driver has to apply a larger pedal force, which increases the driver's burden, and degrades the driver's driving experience.
In addition, U.S. Patent Application Publication No. US20050166748A discloses a dual-rate brake vacuum booster, which mainly differs from the single-rate brake vacuum booster in that an accommodating space is provided at one side of the rubber reaction plate where the base plate of the reaction rod is engaged, and when bearing a large external force, the rubber reaction plate expands and fills up the accommodating space, thereby increasing the action area between the reaction rod and the rubber reaction plate, and hence varying the rate to realize a dual-rate braking. But there are also some disadvantages: the existing dual-rate brake vacuum booster only has an accommodating space between the rubber reaction plate and the base plate of the reaction rod, thus the pressed rubber reaction plate can only expand towards one side, which easily produces an over fatigue and causes a defect of shortened service life.
In view of the defects of the above-mentioned existing dual-rate brake vacuum boosters, the inventor actively makes improvements and innovations based on years of study, the on-site experiences and the professional knowledge, and in conjunction with the practical usages and the technical requirement of the structure, so as to realize a dual-rate brake vacuum booster which is capable of prolonging the service life of the rubber reaction plate.